Improve improvement in the constraint solver
This regrettably-big patch substantially improves the way in which "improvement" happens in the constraint solver. It was triggered by trying to crack Trac #10009, but it turned out to solve #10340 as well. The big picture, with several of the trickiest examples, is described in Note [The improvement story] in TcInteract. The major change is this: * After solving we explicitly try "improvement", by - making the unsolved Wanteds into Deriveds - allowing Deriveds to rewrite Deriveds This more aggressive rewriting "unlocks" some extra guess-free unifications. * The main loop is in TcInteract.solveSimpleWanteds, but I also ended up refactoring TcSimplify.simpl_loop, and its surrounding code. Notably, any insolubles from the Givens are pulled out and treated separately, rather than staying in the inert set during the solveSimpleWanteds loop. There are a lot of follow-on changes * Do not emit generate Derived improvements from Wanteds. This saves work in the common case where they aren't needed. * For improvement we should really do type-class reduction on Derived constraints in doTopReactDict. That entailed changing the GenInst constructor a bit; a local and minor change * Some annoying faffing about with dropping derived constraints; see dropDerivedWC, dropDerivedSimples, dropDerivedInsols, and their Notes. * Some substantial refactoring in TcErrors.reportWanteds. This work wasn't strictly forced, but I got sucked into it. All the changes are in TcErrors. * Use TcS.unifyTyVar consistently, rather than setWantedTyBind, so that unifications are properly tracked. * Refactoring around solveWantedsTcM, solveWantedsAndDrop. They previously guaranteed a zonked result, but it's more straightforward for clients to zonk.
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